In drilling operations a drilling fluid is used, and the drilling fluid has many purposes. For example, drilling fluid may cool and lubricate the drill bit during drilling. The drilling fluid may carry cuttings created downhole back to the surface for disposal. Moreover, the drilling fluid may provide a hydrostatic balance of pressures downhole. That is, the hydrocarbons in a formation have or are under a certain amount of pressure, and the weight of the column of drilling fluid within the borehole provides a counter balancing pressure which prevents the hydrocarbons from escaping to the surface in an uncontrolled manner.
A perfect hydrostatic balance is difficult to achieve, and thus to avoid failure in many cases the density of the drilling fluid is controlled such that the hydrostatic pressure of the drilling fluid downhole is greater than the pressure of the hydrocarbons in the formation. Because of the pressure difference, the drilling fluid may be forced into the formation (i.e., invade the formation). For water-based and oil-based drilling fluids, the invasion does not present significant issues with respect to measuring formation properties (such as density or porosity) because the drilling fluid that actually enters the formation has similar characteristics (e.g., density, photoelectric attenuation coefficient) to the displaced hydrocarbons, and the invasion is to some extent self limiting. However, a new family of drilling fluids is being used in the industry, the new family of drilling fluid comprising “formates” (e.g., cesium formate). Invasion of formates into a formation adversely affect the ability of logging tools to measure formation properties (again, such as density or porosity).